Magnesium. Sample of metallic magnesium made by Sir Humphrey Davy in 1808. Davy made this by the electrolysis of magnesium hydroxide and mercury (II) oxide. Magnesium is an alkaline earth metal in Group II of the periodic table with an atomic number 12 and the symbol Mg. It is a highly reactive metal, used mainly as an alloy with aluminium. This sample is preserved at the Royal Institution, London.